Recently, separation membranes have been utilized in various fields of water treatment, food-stuff industry and the like. In the filed of water treatment such as manufacture of drinking water, water purifying treatment or waste water treatment, a separation membrane is nowadays employed for removing impurities in water in place of a sand filtration process or a flocculation-sedimentation process in the prior process. For the purpose of preventing bio-fouling of the membrane, a sterilizer such as sodium hypochlorite may be added to the membrane module or the membrane itself is washed with an acid, an alkali, chlorine a surfactant or the like, and thus the separation membrane used in a water purifying treatment is also required to have high chemical strength. Furthermore, the separation membrane is required to have high physical strength so as not to cause fracture during use.
As described above, the separation membrane is required to have, in addition to excellent pure water permeability and separation performance, high chemical strength and high physical strength. For this reason, separation membranes formed from a polyvinylidene fluoride-type resin having both the chemical strength and the physical strength, have recently been used.
Also in the fields of manufacture of drinking water, manufacture of pharmaceuticals and food-stuff industry, when pathogens such as viruses are incorporated into the process, since there is a risk that not only a manufacturing line is contaminated, but also multiple infections of consumers arise, various sterilization technologies are used. The sterilization method includes, for example, a heat treatment and a chemical treatment with chlorine or the like, but exerts a slight effect on viruses having thermoresistance and chemical resistance. Thus, membrane filtration using a separation membrane has become of major interest as a method of physically remove viruses. The membrane filtration has many advantages that enable complete removal of viruses, high separation rate, no need for mixing of impurities and the like.
Examples of the smallest actual virus include parvovirus, poliovirus and the like, each having a diameter of 20 to 30 nm, and examples of the pathogenic virus in water include norovirus having a diameter 25 to 35 nm, adenovirus having a diameter of 70 to 90 nm and the like. Various membranes are disclosed as a separation membrane capable of removing such a virus group.
For example, Patent Document 1 describes a hollow fiber membrane for medical use, made of a polyvinylidene fluoride resin, which exhibits high virus removal performance by controlling a maximum pore diameter determined by a bubble point method within a range from 10 to 100 nm and controlling a thickness of a dense structure layer to 50% or more of the entire membrane thickness. However, since the hollow fiber membrane is formed from one layer having a continuous structure including a coarse structure and also has a small membrane thickness, physical strength per one hollow fiber membrane is low and the hollow fiber membrane cannot be applied for water treatment. Also, since the dense layer is too thick, pure water permeability becomes low regardless of a small membrane thickness.
Patent Document 2 discloses a porous membrane made of a polyvinylidene fluoride-type resin which exhibits a porosity of 55 to 90%, a tensile strength of 5 MPa or more and a fracture elongation of 5% or more by adding a high-molecular weight polyvinylidene fluoride-type resin. However, the same document neither describes nor suggests various parameters and mechanisms for the improvement of a virus removal performance. In the case of using the porous membrane made of a polyvinylidene fluoride-type resin having the molecular weight recited in Examples of the same document, sufficient virus removal performance could not be obtained.
Patent Document 3 describes a porous membrane made of polyvinylidene fluoride-type resin, including reinforcing fibers, and a support layer and a dense layer which exhibits a separation characteristic to the support layer. However, the same document also neither describes nor suggests various parameters and mechanisms for the improvement of a virus removal performance. In the case of using the porous membrane made of a polyvinylidene fluoride-type resin recited in Examples of the same document, sufficient virus removal performance could not be obtained.
Patent Document 4 describes a polymeric porous hollow fiber membrane having a characteristic structure, which contains a hydrophobic polymer and a hydrophilic polymer and includes a dense layer on an inner surface and an outer surface, in which a porosity increases toward the outer surface from the inner surface at first and, after passing at least one maximum portion, the porosity decreased at the outer surface side and a pore diameter of the inner surface has a specific relationship with an exclusion limit particle diameter. However, the same document also neither describes nor suggests various parameters and mechanisms for the improvement of a virus removal performance. Also, in Example, there was no description that a polyvinylidene fluoride-type resin was specifically used.    Patent Document 1: Pamphlet of International Publication WO 03/26779    Patent Document 2: Pamphlet of International Publication WO 04/81109    Patent Document 3: JP-A-2002-166141    Patent Document 4: JP-A-2007-289886